Heretofore devices and techniques have been developed for determining the number of telephone sets, if any, connected at any one time to a subscriber telephone line or "loop" as the electrical circuit is termed in the telecommunications industry. Since each telephone set normally has an audible signaling device or "ringer" to alert subscribers to incoming calls, which ringers have significant amount of capacitance, these prior art methods have generally involved a measurement of subscriber loop capacitance as a means for identifying the number of telephone sets. In other words, where two telephone sets are connected to a subscriber loop their ringers will provide twice the capacitance on the loop that would otherwise be provided by one ringer where only one telephone set were to be connected to the loop.
U.S. Pat. No. 3,870,837 exemplifies one such prior art system and method wherein first and second frequency signals are applied to a subscriber loop with the first frequency being lower than the ringing device resonant frequency and the second frequency being higher than the ringing device resonant frequency. The current resulting from the coupled first and second signals is obtained from a current sensing device in series with the loop. The first frequency current is a measure of the capacitance of the loop and the capacitance of the ringers connected thereto since the admittance of the ringers is capacitive below resonance. The second frequency current is a measure of the loop capacitance only since the ringer admittance well above resonance is small compared to the loop admittance.
U.S. Pat. No. 4,022,990 describes another technique which, through relatively complex circuitry, seeks to eliminate certain interference and line capacitance not attributable to ringers on the subscriber loop in order to provide a more accurate reading. This is accomplished here through the use of a test capacitor to which is transferred any charge induced into the unknown ringer capacitor on the subscriber loop. By measuring the transferred charge the unknown capacitance can be determined.
U.S. Pat. No. 4,054,760 provides yet another system which also seeks to eliminate or reduce the presence of capacitance in a subscriber's loop not attributable to telephone set ringers themselves. This system employs a reversible switch located between the central office and subscriber's location. Testing is accomplished by locating the reversing switch at a place remote from the central office but relatively close to the subscriber's station. By metering changes in current level on the subscriber's loop while reversing the switch a more accurate measure may be obtained of that capacitance on the subscriber loop attributable to the telephone set ringers.
The just described procedures have been used to ascertain the signaling load on the subscriber loop or to detect the existence of unauthorized in-service telephone sets. This testing has been accomplished at the central office itself. Once service has been discontinued, however, there may still remain the need to ascertain whether or not one or more telephone sets are connected to the loop. Due to a conventional division of telephone company organizational responsibilities it typically is delegated to a group of telephone field workers to recover telephone sets that are no longer in use on subscriber loops. It often happens, however, that when a telephone company representative arrives at the subscriber's location he cannot readily gain access to the interior of the building in which the subscriber is located to retrieve the telephone sets. This is a common predicament due to the fact that a discontinuance of service typically accompanies a relocation of the subscriber from one building or house to some other. When this occurs the telephone field representative must return at a later time to the building after having acquired entry authorization, door keys and the like to make his inspection. Still then however he often will find no telephone set located within the building on the subscriber's out-of-service loop. This, of course, makes the return trip unproductive other than to have ascertained the absence of any set within the subscriber's building or home. Furthermore, even once he has gained entry he cannot, upon finding one telephone set, assume that that is the only set in the building. Rather, he must inspect each individual room in the building.
Though it is possible, as previously described, for the central office to conduct a test to determine the presence of ringers and thus telephone sets within a subscriber's building, this can be an inefficient procedure due to the fact that testers at central offices are themselves not charged with the responsibility of recovering out-of-service phones. Thus, in using this procedure the field unit charged with that responsibility must contact a central office tester who has to have service temporarily reestablished to the subscriber loop in order to conduct the test. At the conclusion of the test the field unit, which has either been standing idly by or since moved to another location and job, is advised.
It thus is seen that there remains a need for apparatuses and methods for determining the presence of out-of-service telephone sets within a building which could be readily used by field representatives of the telephone company without having to contact the central office and temporarily reestablish subscriber service. It is to this task to which the present invention is primarily directed.